1. Field of the Invention
The present invention relates to a photonic crystal device, and in particular to a photonic crystal device which includes an optical waveguide and an optical resonator.
2. Description of the Related Art
In a periodic refractive index modulation structure which is formed within a dielectric or a semiconductor, electromagnetic waves such as light are susceptible to periodic perturbations. Therefore, a light band structure (“photonic band structure”) whose relationship between wave number and frequency (dispersion relationship) is similar to that of a band structure of electrons within a crystal is formed. Such a periodic refractive index modulation structure is called a photonic crystal (J. D. Joannopouls et al., “Photonic crystals”, Princeton University Press, 1995). Light propagation in a photonic crystal can be controlled based on the material and the photonic crystal structure.
Optical waveguides to which a photonic crystal structure is applied are attracting much attention as a technique which enables downsizing of optical circuit devices. In an optical waveguide which utilizes a photonic crystal structure, what is important is the refractive index difference between the optical waveguide portion and the periodic structure portion. Therefore, there have been many reports of examples of photonic crystals produced from a combination of a high refractive index material whose refractive index is 3 or more, such as gallium arsenide (refractive index: 3.6) or silicon (refractive index: 3.4), and a low refractive index material such as silicon dioxide (refractive index: 1.5) or air (refractive index: 1) (see, for example, Japanese Laid-Open Patent Publication No. 2002-350657).
Chutinan et al., Physical Review B, vol. 62, No. 7, p4488, 2000, discloses an optical waveguide in which a photonic band is created by forming a periodic array of cylindrical air holes in a semiconductor substrate, thus providing an optical path which is bent perpendicularly. Y. Akahane et al., “Investigation of high-Q channel drop filters using donor-type defects in two-dimensional photonic crystal slabs”, Applied Physics Letters, vol. 83, p. 1512, 2003, discloses an optical resonator of a size of a light wavelength, which is produced by utilizing a photonic crystal. Y. Akahane et al., “Fine-tuned high-Q photonic crystal nanocavity”, OPTICS EXPRESS, vol. 13, No. 4 p. 1202, 2005, discloses a photonic crystal device which includes an optical resonator and an optical waveguide.
Hereinafter, with reference to FIG. 1, a conventional photonic crystal device which is produced by using a photonic crystal structure will be described.
FIG. 1 is an upper plan view of a two-dimensional photonic crystal device. In this two-dimensional photonic crystal device, a two-dimensional photonic crystal is formed by arraying a multitude of air holes 102 in a semiconductor (silicon) layer of an SOI (Silicon-On-Insulator) substrate 101. The photonic crystal has a photonic band structure which prevents propagation of light within a specific wavelength band. A photonic crystal device as shown in FIG. 1 can be produced by, for example, arraying air holes (having a diameter of 240 nm) at intervals of 420 nm on a slab (having a thickness of 250 nm), so as to form a triangular lattice.
The substrate 101 has a linear portion in which no air holes 102 are formed, the linear portion functioning as an optical waveguide 103. The optical waveguide 103 is a line defect in the photonic crystal. That is, no photonic band structure is present in the optical waveguide 103.
At a position which is at least one air hole 102 away from the optical waveguide 103, there exists a space 104 in which no air holes 102 are formed. This space 104 consists of point defects in the photonic crystal, and functions as a very small sized optical resonator, having a size on the order of a light wavelength. The optical resonator length is an integer multiple of the diameter of the air holes 102. Out of the light which propagates through the optical waveguide 103, light within a specific wavelength region resonates in the space 104. Since the space 104 is enclosed by the surrounding photonic crystal, the space 104 can exhibit a high Q value as an optical resonator. The illustrated photonic crystal device can be utilized in various devices such as optical filters and semiconductor lasers.
A diagram (FIG. 5) from Y. Akahane et al., “Fine-tuned high-Q photonic crystal nanocavity”, OPTICS EXPRESS, vol. 13, No. 4 p. 1202, 2005, supra, is shown in FIG. 11. FIG. 11 is a graph whose vertical axis represents optical intensity (Intensity: arbitrary unit) and whose horizontal axis represents wavelength (Wavelength: nm). In FIG. 11, (a) shows the intensity of light which is transmitted and propagates through the optical waveguide, whereas (b) shows the intensity of light which is radiated to the outside through the optical resonator. The light which is radiated to the outside of the substrate through the optical resonator has a narrow spectrum at a resonant wavelength, the resonant wavelength being controllable by adjusting the resonator length. In such a photonic crystal device, by adjusting the degree of coupling between the optical resonator and the optical waveguide, it becomes possible to control the intensity of the light which is radiated to the outside through the optical resonator.
In the conventional photonic crystal device shown in FIG. 1, the resonant frequency is controlled by adjusting the length of the space 104 functioning as an optical resonator. On the other hand, the degree of optical coupling (“matching”) between the optical resonator (space 104) and the optical waveguide 103 is adjusted based on the distance between the space 104 and the optical waveguide 103. However, this distance is set to be an integer multiple of the pitch of the air holes (lattice constant), and therefore it is difficult to realize a precise adjustment of the degree of optical coupling. Therefore, it has been impossible to arrive at a design which simultaneously realizes desired values for the Q value, matching, and resonant frequency of the optical resonator.